Total gaseous mercury exchange between air and water at river and sea surfaces in Swedish coastal regions

This study includes five intensive field measurement campaigns. Four of the campaigns were performed over seawater surface during the summer and winter of 1997 and the summer of 1998 at Kristineberg Marine Research Station (KMRS). The fifth campaign was conducted over a river surface during the summer of 1999 at Knobesholm in southwestern Sweden. The major purpose of these campaigns was to determine emissions of mercury from natural waters in northern Europe. The influence of some physical parameters, i.e. temperature in water and air, relative humidity and solar radiation were also examined. Dynamic flux chamber technique coupled with automatic mercury vapour-phase analysers (Gardis 1A or Tekran 2357) was used. Both sites show net evasion during summer season, however, the surficial evasion rate of the river is more than one order of magnitude higher than that of the seawater. The high content of organic matter in the river in conjunction with strong insolation and subsequent water temperature variations may explain the high mercury evasion measured at the river site. An average evasion of +11 ngm−2 h−1 (varying from −2.5 to +88.9 ngm−2 h−1) was obtained during the course of the river measurement. At the sea site, mercury evasion was found in the interval between −2.72 and +8.84 ngm−2 h−1 with an average evasion of +0.61 ngm−2 h−1. Mercury evasion measured over both river and seawater surfaces exhibits a consistently diurnal pattern with maximum evasion during the daytime period and minimum evasion during the nighttime period. At the freshwater site, mercury evasion is strongly correlated with the intensity of net insolation, and negatively correlated with relative humidity. An exponential relationship between mercury evasion and water temperature was also observed at the freshwater measurement site. At the seawater site, a strong correlation between mercury evasion and intensity of UVA part of insolation was obtained. Insolation is speculated to play an important role in the formation of dissolved gaseous mercury in both river and seawater.